How to set minimum and maximum fields of a custom ID property using Python
This is what you do to set min to 0 and max to 500, the soft_min and soft_max is optional and is only for dragging in gui:

ob = bpy.data.objects[name_of_my_object]

# this is a dictionary containing IDprops settings
# this creates it and erases it:
ob["_RNA_UI"] = {}

# add properties settings like this:
ob["_RNA_UI"]['name_of_my_property'] = {"min":0.0,
                                        "max": 500.0,
                                        "soft_min":0.0,
                                        "soft_max":500.0}